1. Field of Invention
This invention relates broadly to hydraulic circuits for sequencing the actuation of hydraulic cylinders. More specifically, this invention relates to a hydraulic circuit for sequentially retracting and extending a pair of hydraulic cylinders.
2. Description of the Prior Art
A variety of operations that are performed by hydraulically powered motors are facilitated by the automatic sequencing of the hydraulic motors. One such application is the operation of row markers for multi-row farm implements. A row marker is used to guide a tractor in successive passes during a planting or cultivating operation. Typically, the row marker projects to the side of the tractor by a distance equal to the width of the implement the tractor is pulling. Thus as the tractor proceeds up the field, it makes a center mark for the succeeding pass. When the tractor comes to the end of a row, the marker in use is raised and a marker extending to the opposite side of the tractor is lowered to again mark the path for the next pass. In order to simplify operation, alternate raising and lowering of both markers is done using a single control having a raised or lowered position. A sequencing circuit automatically lowers and raises each marker sequentially in response to input from the control.
Systems for sequencing the actuation of hydraulic cylinders are well known in the art. A system specifically suited for sequencing row markers is shown in U.S. Pat. No. 4,285,268 issued to Deckler. This patent shows a pair of hydraulic cylinders actuated by a manual control to be raised and lowered sequentially by an automatic sequencing valve. It is commonly practiced in such systems that movement of the automatic sequencing valve is controlled by sensing a pressure difference between the loading lines to each hydraulic cylinder. In order to provide a suitable pressure differential for shifting the valve, the entire flow of fluid for loading the cylinders passes through a flow restrictor. It is difficult to size a simple orifice for the full range of flow rates and oil viscosities that the mechanism may be exposed to during operation. Accordingly, the orifice is either somewhat undersized, thereby slowing down the function of the hydraulic cylinders, or oversized, resulting in improper sequencing at low flow rates. Deckler also exemplifies the common practice of using a spool valve as the load holding device for the hydraulic cylinders, which increases the susceptibility of such a system to leak down.
Accordingly, it is an object of this invention to provide a hydraulic sequencing circuit that will provide quick response and proper sequencing over a wide range of operating conditions.
It is a further object of this invention to provide a hydraulic sequencing circuit that minimizes the possibility of leak down from the load holding elements.
It is yet a further object of this invention to provide a valve for a hydraulic sequencing circuit that will provide the restriction function necessary for proper operation.